FIG. 1 shows the configuration of a mobile communication system including a conventional UTRAN (Universal Terrestrial Radio Access Network).
With reference to FIG. 1, description is given of an operation which is performed when an “S-RNC (Serving Radio Network Controller) Relocation” occurs in the above-described mobile communication system.
As shown in FIG. 1, in the mobile communication system, the “S-RNC Relocation” of a mobile station UE (User Equipment) from a radio controller RNC#1 to a radio controller RNC#2 occurs at step S101, and thereafter, an exchange XGSN transmits a “NAS-PDU (Non Access Stratum-Protocol Data Unit)” to the radio controller RNC#1 at step S102. In this case, the radio controller RNC#1 discards the NAS-PDU at step S103.
Non-Patent Document 1: 3GPP TS25.413 UTRAN Iu Interface RANAP Signaling
Non-Patent Document 2: 3GPP R3-062003 NAS Signaling in E-UTRAN
Non-Patent Document 3: R3-070700 LS on NAS handling during intra-LTE handover
As described above, in the conventional mobile communication system, the radio controller RNC#1 discards the NAS-PDU transmitted from the exchange XGSN during execution of an “S-RNC Relocation” of the mobile station UE from the radio controller RNC#1 to the radio controller RNC#2. Thus, there is a problem that the exchange XGSN cannot retransmit the NAS-PDU to the radio controller RNC#2.
Moreover, a mobile communication system based on the LTE (Long Term Evolution) standard has the same problem. During handover of a mobile station UE, a handover source radio base station eNB fails to transmit a NAS-PDU received from an exchange MME (Mobility Management Entity) to the mobile station UE, and discards the NAS-PDU. Consequently, the exchange MME cannot retransmit the NAS-PDU to a handover target radio base station eNB.